MINOR: ssl: fix possible null dereference in error handling

MINOR: ssl: fix possible null dereference in error handling

recent commit 8b453912ce9a ("MINOR: ssl: ssl_sock_prepare_ctx() return an error code")
converted all errors handling; in this patch we always test `err`, but
three of them are missing. I did not found a plausible explanation about
it.

this should fix issue #374

Fixes: 8b453912ce9a ("MINOR: ssl: ssl_sock_prepare_ctx() return an error code")
Reported-by: Илья Шипицин <chipitsine@gmail.com>
Signed-off-by: William Dauchy <w.dauchy@criteo.com>